Castiel (Misha Collins) is an angel on "Supernatural." He has an emotionless demeanor and rarely helps humans. He follows his orders like a loyal soldier and would gladly take an innocent's life to achieve his goal, though as the seasons progress, he is seen growing fond of the Winchester brothers -- particularly Dean Winchester (Jensen Ackles). Castiel also defies God's orders to help Dean.
Castiel first appeared raising Dean from the dead in the season 4 premiere "Lazarus Rising." He was instantly met with distrust as the Winchester brothers had no knowledge that angels existed. Castiel takes a human form, as his natural form and voice would cause blindness and deafness to a human who isn't a prophet.
Castiel was faced with the task of helping Sam Winchester (Jared Padalecki) and Dean Winchester stop Lilith (various characters) -- a powerful demon -- from breaking the 66 seals which keep Lucifer bound in hell. They ultimately failed in their quest to stop Lilith from breaking the seals, and Lucifer rises from hell. Castiel aided the Winchester brothers in defeating Lucifer, giving his knowledge and expertise to assist them.
Castiel defied his superiors and helped Dean escape a heavenly prison after he found out the angels wanted the apocalypse to happen. He died in the season 5 finale battling Lucifer, but God resurrected him.
Dean: [Castiel is about to "transport" him and Dean to a different location] Whoa! Whoa!
Dean: The last time you zapped me some place, I didn't poop for a week!
Dean: We're driving!